OK hive, I'm stumped (a bit).
On my Fmod project I am working on the fuel tank. I am using the kart tank from when we ran the 440 motor and the Yamaha pump assembly. To hold everything together and get the best seal, I made a ring for inside the tank. The ring is steel and I used M6 bolts, heads ground off and welded to the ring. When test fitting, two of the six bolts snapped below the welds. (Cheap @ss screws?) Instead of trying to save it, I am remaking the part. This time do I want to leave the heads on the bolts? If so, do I want to just tack them or weld around the head for a better seal?
In reply to triumph7 :
I don't know if I totally get it, but could you reverse the bolts and put rivnuts in the ring? With rubber washers under the bolts?
Well, aside from not having rivnuts I need the bolts to come from inside to hold the ring in place without more holes to hold the ring.
Honsch
Reader
5/6/21 9:46 p.m.
Two things.
What filler did you use to attach the SS screws ro the mild steel ring? For mixed alloys, 309 is a good choice.
Did you use SS nuts? What lube was on them? SS will gall like nobody's business if you use the same alloy nuts and bolts.
I'd say leave the heads in place. Put an oring on the threaded side of each bolt as your seal.
In reply to Honsch :
No stainless screws, metric. Just regular steel.
In reply to wvumtnbkr :
That's what I was thinking except for the o-rings. I think that without the heads that too much heat went into the screws and softened them. With the heads on they should do better. O-rings will be too thick to let the gasket seal.
I might even use flange-head bolts rather than regular bolts, just to give more surface area for the weld.
How bout some gasoline resistant rtv on those bolts? Sshould be okay.
In reply to wvumtnbkr :
Could work but I'd feel better with the heads welded. I tried doing RTV without welding but a couple of the screws backed out as they were being tightened.
There's a surprising amount of load going thru bolts, and welds in tension aren't a great idea. If you essentially made the bolts into studs and welded those to the ring - the weld is both reacting the assembly torque and the tension load.
Drill the flange, and tack weld the head of the bolt on the back side. The weld is only going to react the torque load which is pretty low on an m6 bolt. Should work wonderfully.
In reply to Brotus7 :
Yeah, I'm probably over-concerned about leakage around the bolt head. Interesting point, the failure wasn't at the weld, it was a couple threads down.
What brotus said.
Welding a fastner nukes the heat treatment and however you are letting it cool will likely not be predictable enough to rely on. Minimum heat tack weld in the way brotus mentions while still getting penetration for the tack. Don't quench!
For sealing use something else. Some product from a tube rated for gas.
Other option is a thicker phlange, drilling and tapping and getting thread in studs from some place like McMaster.
If you're worried about leakage around the bolt heads, you could encapsulate the bolt heads in JB weld after to tack them. In addition, I'd use something like Motoseal or another fuel safe rtv on the flange when you bolt it all together.
Well, I tacked the bolts, put a thick gasket on it and assembled everything. Seems to be leak free as long as I don't put it under pressure. Even with some air pressure I just get very minor bubbling (soapy water on the outside). With fuel it looks good.
